(a) Field of the Invention
The present invention relates to sampling frequency generation. More particularly, the present invention relates to a method and apparatus for generating a sampling frequency used for signal processing in mobile communication.
(b) Description of the Related Art
In a communication system, a transmitting device converts baseband digital data into an analog baseband signal using a DAC (digital-to-analog converter), and up-converts the analog baseband signal into an RF (radio frequency) center frequency and transmits it. Such an RF transmission signal is received by a receiving device. The receiving device down-converts the received RF signal into an analog baseband signal, and samples the analog baseband signal and converts it into a digital baseband signal using an ADC (analog-to-digital converter).
For ideal communication, sampling frequency must satisfy a predetermined condition. That is, the clock of the DAC of the transmitting device and the clock of the ADC of the receiving device must match each other. If this condition is not satisfied, aliasing can occur in a particular frequency range depending on the bandwidth of a baseband signal according to the Nyquist Sampling Theorem, causing a problem in proper data transmission. Normally, the operation of measuring such frequency error is done in the digital domain.
However, it is very difficult to measure sampling frequency errors unless the analog domain supports specific hardware, because the clock of the digital domain is generally used as a sampling clock. Therefore, the receiving device must be initially designed to make sure that the analog domain provides as accurate values of sampling frequency as possible at the time of sampling frequency generation. However, the receiving device (e.g., a terminal) generally uses a reference clock, which is less accurate. This limits the ability to provide accurate values of sampling frequency.
Accordingly, there is a need to employ a negative feedback loop which uses a base station's clock as a criterion, in order to generate a highly accurate and stable clock. However, since an RF center frequency error compensation circuit is already realized in the negative feedback loop, a sampling frequency generation circuit is generally designed to use the RF center frequency error compensation circuit.
That is, the system is designed in such a way that both sampling frequency and RF center frequency, which are set to be multiples of each other, are multiples of the terminal's reference clock. Also, the sampling frequency can be compensated automatically once an RF frequency error is measured and compensated for, which is a widely-used method.
Using this method, the RF center frequency is synchronized with a signal received by the terminal, but the sampling frequency is different from the value received by the base station, which is likely to cause aliasing. Particularly, sampling frequency changes in proportion to frequency error occur in a channel, caused by the Doppler effect. The higher the sampling frequency and the moving velocity of the terminal, the greater the amount of change in sampling frequency. In view of the recent trend toward higher communication throughput by the use of higher bandwidth and toward support for higher mobility, it is not appropriate to generate sampling frequency based upon the above method.
Normally, a modem receives a sampling frequency and uses it as a clock signal for a digital circuit. However, this method involves designing the digital circuit in view of changes in sampling frequency in a high-velocity environment, which can be a significant burden to design.
In addition, if a guard band is not large enough compared with sampling frequency errors, aliasing will occur. In the worst case, aliasing may occur in a frequency band where channel distortion measurement is made, interfering with proper frequency synchronization.
Accordingly, it is difficult to get a stable sampling frequency under a high-velocity mobile environment only by measuring an error and compensating for it as described above, while maintaining sampling frequency and RF center frequency to be multiples of each other.
Therefore, generating a sampling clock in a way that it is kept at a constant frequency, regardless of moving velocity, rather than by the above-described method, will get the modem closer to a more ideal design solution. To this end, a new method and apparatus for achieving sampling frequency that are different from the existing ones are needed.